Grand Challenges in Sustainable Ammonia Synthesis – DOE Roundtable Report, 2016

Earlier this year, the US Department of Energy (DOE) hosted a day-long meeting “to explore the scientific challenges associated with discovering alternative, sustainable processes for ammonia production.”

The report that came out of this roundtable discussion presents the participants’ views on “the current state-of-the-art and the potential challenges and research opportunities … for heterogeneous catalysis and homogeneous and enzyme catalysis.”

The participants included a dozen leading academics from the US, and one from Denmark.

They concluded that we must overcome a series of hurdles in the development of alternative, sustainable technologies for ammonia synthesis – but one particular issue was highlighted as their “overarching grand challenge.”

Discovery of active, selective, scalable, long-lived catalysts for sustainable ammonia synthesis

As the report begins, “in spite of many new insights, the industrial catalyst used today is surprisingly similar to the original one discovered by Mittasch in the beginning of the 20th century.” It concludes: “Currently there is no viable heterogeneous, homogeneous, or enzyme catalyst known that fulfills all of the requirements for an active, selective, scalable, long-lived catalyst. This is true for homogenous redox or electrochemical processes, as well as for electrochemical or photochemical surface processes.”

This entails integrating theory and experiment to understand mechanisms and identify catalyst descriptors that can be used in catalyst design. It also requires integrating knowledge from heterogeneous, homogeneous, and enzyme catalysis. A number of new tools in computational and in-situ and operando characterization of catalysts also need to be invoked to ensure success.
DOE Roundtable Report, Sustainable Ammonia Synthesis, February 2016

The roundtable produced 7 other “grand challenges and research opportunities.”

Development of relatively low pressure (<10 atm) and relatively low temperature (<200 C) thermal processes
Given the possibility of delocalized dihydrogen production through electrolysis or a photoelectrochemical device, it is desirable to find alternative catalysts that can enable a thermal process at relatively low temperature. Such a catalyst opens the door to developing a low- pressure process compatible with low-pressure dihydrogen generation and small-scale production.

Development of electrochemical and photochemical routes for N2 reduction based on proton and electron transfer
Essential to most non-thermal alternatives to the Haber-Bosch process, it is critical to develop electrochemical processes for N2 reduction using electrons generated from solar or wind resources. There is a need to build upon molecular level mechanisms for electrochemical and photochemical N2 reduction and the associated energetics that were demonstrated on molybdenum-based catalysts.

Development of biochemical routes to N2 reduction
New, creative means to functionally immobilize recalcitrant redox enzymes, including nitrogenase, on electrode surfaces will open new possibilities for feeding sustainable energy into a biological process.

Development of chemical looping (solar thermochemical) approaches
Fundamental understanding is required to identify new alloys and doped materials and to achieve process integration and optimization for the N2 activation through metal nitridation/reduction cycles using heat/photons generated by solar thermal processes.

Identification of descriptors of catalytic activity using a combination of theory and experiments
Given that there has been only limited success so far in identifying new catalysts that could form the basis for sustainable ammonia production, new catalyst design approaches are needed. Computational methods should be combined with atomic-scale controlled synthesis, operando characterization, and catalytic testing. A key element is the identification of the most important descriptors of catalytic activity and selectivity.

Integration of knowledge from nature (enzyme catalysis), molecular/homogeneous and heterogeneous catalysis
Insight from heterogeneous, homogeneous and enzyme catalysis, if combined, could strengthen the possibilities for a breakthrough in catalyst design. The field of ammonia synthesis could provide a “test bed” for a more integrated approach to catalyst discovery in general.

Characterization of surface adsorbates and catalyst structures (chemical, physical and electronic) under conditions relevant to ammonia synthesis
There is a strong need to invoke all the new tools in computational and in situ and operando characterization of catalysts. This includes chemical, physical, and electronic characterization of the active catalyst, reaction intermediates and the relevant bond energies, as well as the effects of reaction media, such as gas, solution, ionic liquid, or electrolyte.
DOE Roundtable Report, Sustainable Ammonia Synthesis, February 2016

This article can also be found over at ammoniaindustry.com.